Devices such as a cellular phone, a digital camera, a PDA, a touch panel display, a large-screen television, and a wireless charging system tend to be more widely used.
In any of those applications, a resin substrate such as an acrylic substrate has been conventionally used as a protective member for protecting a display. However, the resin substrate has a low Young's modulus, and hence, when the view surface of the display was pushed with a pen, a human finger, or the like, the resin substrate easily bended and touched with the internal display, sometimes causing a display defect. Further, the resin substrate has also involved a problem in that the resin substrate is liable to have flaws on its surfaces, resulting in easy reduction of its visibility. One of the ways to solve those problems is to use a glass substrate as a protective member. The glass substrate (cover glass) used as a protective member is required to, for example, (1) have a high mechanical strength, (2) have a low density and a light weight, (3) be able to be supplied at low cost in large quantity, (4) is excellent in bubble quality, (5) have a highlight transmittance in the visible region, and (6) have a high Young's modulus so as not to bend easily when its surface is pushed with a pen, a finger, or the like. In particular, when a glass substrate does not satisfy the requirement (1), the glass substrate does not satisfactorily serve as a protective member, and hence a glass substrate tempered by, for example, ion exchange (which is the so-called tempered glass substrate) has been conventionally used as a protective member (see Patent Literatures 1 and 2, and Non Patent Literature 1).